Low loss power device and method for fabricating thereof

ABSTRACT

Existing semiconductor transistor processes may be leveraged to form lateral extensions adjacent to a conventional gate structure. The dielectric thickness under these lateral gate extensions can be varied to optimize device channel resistance and enable resistance to breakdown at high operating voltages. These extensions may be patterned with dimensions that are not limited by lithographic resolution and overlay capabilities and are compatible with conventional processing for ease of integration with other devices. The lateral extensions and dielectric spacers may be used to form self-aligned source, drain, and channel regions. A thin dielectric layer may be formed under an extension gate to reduce channel resistance. A thick dielectric layer may be formed under an extension gate to improve operation voltage range. The present invention provides an innovative structure with lateral gate extensions which may be referred to as EGMOS (extended gate metal oxide semiconductor).

The present application for patent claims priority to pendingprovisional application No. 63/032,159, titled “IntegratedSilicon-on-Insulator (SOI) Devices Suitable for Radio Frequency (RF)Applications,” filed on May 29, 2020, and assigned to the assigneehereof and hereby expressly incorporated by reference herein as if fullyset forth below and for all applicable purposes.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device. Moreparticularly, the present invention relates to a low loss power deviceand a method for fabricating thereof.

BACKGROUND OF THE INVENTION

Transistors are frequently used as elements in switches and amplifiers.Silicon-based metal-oxide-semiconductor field effect transistor (MOSFET)technology has been refined for decades to improve high-frequencyperformance and reduce costs. Silicon MOSFETs are particularlyattractive for their relative ease of fabrication and large-scaleintegration facilitated by the high demand for microprocessors, as wellas their power-efficiency compared to older bipolar transistor devices.Variations on conventional low-power MOSFETs used in microprocessorshave been developed for use in high-power and high-frequencyapplications. Improved doping profiles and device geometries may be usedto allow for operation at voltages of tens of volts or more. Modifiedmaterial systems such as silicon-on-insulator technology and III-Vsemiconductor technology may be used to achieve higher power operationand higher maximum operating frequencies through improved carriermobility.

SUMMARY OF THE INVENTION

This paragraph extracts and compiles some features of the presentinvention; other features will be disclosed in the follow-up paragraphs.It is intended to cover various modifications and similar arrangementsincluded within the spirit and scope of the appended claims.

The following presents a simplified summary of one or more aspects ofthe present disclosure, in order to provide a basic understanding ofsuch aspects. This summary is not an extensive overview of allcontemplated features of the disclosure, and is intended neither toidentify key or critical elements of all aspects of the disclosure norto delineate the scope of any or all aspects of the disclosure. Its solepurpose is to present some concepts of one or more aspects of thedisclosure in a simplified form as a prelude to the more detaileddescription that is presented later.

Existing semiconductor transistor processes may be leveraged to formlateral extensions adjacent to a conventional gate structure. Thedielectric thickness under these lateral gate extensions can be variedto tune device performance and enable higher cut-off frequencies withoutcompromising resistance to breakdown at high operating voltages. Theseextensions may be patterned with dimensions that are not limited bylithographic resolution and overlay capabilities and are compatible withconventional processing for ease of integration with other devices. Thelateral extensions and dielectric spacers may be used to formself-aligned source, drain, and channel regions. A narrow-highly-dopedchannel may be formed under a narrow gate extension to improve operatingfrequencies without significantly increasing gate capacitance

In one aspect, the present invention provides a low loss power deviceincluding: a substrate, having a semiconductive region extending below atop surface of the substrate, the semiconductive region having first andsecond ends opposing one another along a direction parallel to the topsurface of the substrate; a first dielectric layer, formed above thesemiconductive region of the substrate, having a first thickness; afirst gate electrode, disposed on the first dielectric layer over thesemiconductive region between the first and second ends; a seconddielectric layer, having a second thickness, formed on the top surfaceof the substrate adjacent to the first dielectric layer below the firstgate electrode and near the first end, wherein the second dielectriclayer is formed separately after the first dielectric layer is formed;and a second gate electrode, disposed over the second dielectric layerand in electrical contact with the first gate electrode, wherein asidewall of the second gate electrode opposite the first gate electrodeis substantially perpendicular to a top surface of the substrate.

Preferably, the second thickness is not equal to the first thickness.

Preferably, the second thickness is less than the first thickness.

Preferably, the low loss power device further includes a first spacer,formed above the semiconductive region of the substrate and adjacent tothe second gate electrode.

Preferably, the low loss power device further includes anelectrically-conductive layer formed on the first dielectric layer andthe second dielectric layer.

Preferably, the low loss power device further includes a third gateelectrode adjacent to the first gate electrode disposed over thesemiconductive region near the second end and separated from thesemiconductive region by a third dielectric layer, wherein the thirddielectric layer has a third thickness.

Preferably, the second thickness is less than the first thickness andthe third thickness; and the first thickness is less than the thirdthickness.

Preferably, a sidewall of the third gate electrode opposite the firstgate electrode is substantially perpendicular to a top surface of thesubstrate.

Preferably, the low loss power device further includes a second spacer,formed above the semiconductive region of the substrate and adjacent tothe third gate electrode.

Preferably, a doped source well is formed within the semiconductiveregion at the first end; a doped drain well is formed within thesemiconductive region at the second end; and a doped channel is incontact with the doped source well at an end of the doped source welldistal from the first end of the semiconductive region, the dopedchannel well is disposed beneath the second gate electrode and separatedfrom the second gate electrode by the second dielectric layer, the dopedchannel well has a majority carrier type opposite a majority carriertype of the doped source well and the doped drain well.

Preferably, the low loss power device further includes a doped driftregion extending within the semiconductive region between the dopedchannel well and the doped drain well, the doped drift region isdisposed beneath the first gate electrode and separated from the firstgate electrode by the first dielectric layer, the doped drift region hasa majority carrier type opposite the majority carrier type of the dopedchannel and has a majority carrier density lower than majority carrierdensities of the doped channel, the doped source well, and the dopeddrain well.

Preferably, the low loss power device further includes a graded dopingprofile between the doped channel and the doped drift region.

Preferably, the low loss power device further includes anelectrically-conductive material formed on the first gate electrode andthe second gate electrode to electrically couple the first gateelectrode and the second gate electrode.

In another aspect, the present invention provides a method forfabricating the aforementioned low loss power device. The methodincludes the steps of: providing a substrate having a semiconductiveregion extending below a top surface of the substrate, thesemiconductive region having first and second ends opposing one anotheralong a direction parallel to the top surface of the substrate; forminga first dielectric layer above the semiconductive region; disposing afirst gate electrode over the semiconductive region between the firstand second ends; forming a second dielectric layer having a secondthickness on a first region of the top surface of the substrate adjacentto the first dielectric layer below the first gate electrode and nearthe first end; and disposing a second gate electrode over the seconddielectric layer.

Preferably, the method further includes after the first gate electrodeis disposed and before the second dielectric layer is formed the stepsof: forming a third dielectric layer over the first gate electrode andthe first dielectric layer; and patterning the first and thirddielectric layers to expose the first region of the top surface of thesubstrate.

Preferably, the method further includes a step of: applying a firstdopant to form a first doped volume within a first volume of thesubstrate corresponding to the first region, the first doped volumehaving a width determined at least in part by a width of the firstregion.

Preferably, the method further includes the steps of: forming a firstspacer adjacent to the second gate electrode; and applying a seconddopant to form a second doped volume within the first doped volume ofthe substrate, the second doped volume having a width determined by atleast a width of the first region, a position of the second gateelectrode, and a width of the first spacer; wherein the second dopedvolume has a majority carrier type opposite to a majority carrier typeof the first doped volume.

Preferably, the second gate electrode is formed by the steps of: formingan electrically-conductive layer above the first gate electrode and thefirst region; and patterning the electrically-conductive layer by ananisotropic reactive ion etching (RIE) process that leaves behind aportion of the electrically-conductive layer on the second dielectriclayer to form the second gate electrode, the second gate electrodehaving vertical sidewalls that are substantially perpendicular to thetop surface of the substrate.

Preferably, the electrically-conductive layer is patterned after formingabove the first gate electrode and above the first region withoutforming any layer that acts as a mask defining dimensions of the secondgate electrode; and a width of the second gate electrode is defined byan as-formed thickness of the electrically-conductive layer.

Preferably, patterning the electrically conductive layer comprises thesteps of: forming a protective dielectric material layer above theelectrically-conductive layer with a protective layer thickness; andetching the protective dielectric material layer using the anisotropicRIE process; wherein the anisotropic RIE process preferentially etchesthe protective dielectric layer and the electrically-conductive layeralong a direction perpendicular to the top surface of the substrate;wherein the anisotropic RIE process removes the protective dielectriclayer with a greater etching rate than an etching rate for theelectrically-conductive layer; wherein the protective layer thicknessand the anisotropic RIE process are jointly configured such thatresidual protective dielectric material adheres to a vertical sidewallof the second gate electrode farthest from the first gate electrode thatfaces the first gate electrode; and wherein the protective layerthickness and the anisotropic RIE process are jointly configured suchthat residual protective dielectric material adheres to a verticalsidewall of the third gate electrode farthest from the first gateelectrode that faces the first gate electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic view of an example deviceaccording to embodiments disclosed herein.

FIG. 2 is a cross-sectional schematic view of another example deviceaccording to embodiments disclosed herein.

FIGS. 3A-3F are cross-sectional schematic views depicting a device suchas the device of FIG. 1 at different stages of fabrication according toembodiments disclosed herein.

FIG. 4 is a flow diagram of an example transistor fabrication processaccording to some embodiments.

FIG. 5A is a cross-sectional schematic view of another example deviceaccording to embodiments disclosed herein.

FIG. 5B is cross-sectional schematic view illustrated elements of afabrication process of the device of FIG. 5A according to embodimentsdisclosed herein.

FIG. 5C is a cross-sectional schematic view of another example deviceaccording to embodiments disclosed herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more specifically withreference to the following embodiments.

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various configurations and isnot intended to represent the only configurations in which the conceptsdescribed herein may be practiced. The detailed description includesspecific details for the purpose of providing a thorough understandingof various concepts. However, it will be apparent to those skilled inthe art that these concepts may be practiced without these specificdetails. In some instances, well known structures and components areshown in block diagram form in order to avoid obscuring such concepts.

Within the present disclosure, the word “exemplary” is used to mean“serving as an example, instance, or illustration.” Any implementationor aspect described herein as “exemplary” is not necessarily to beconstrued as preferred or advantageous over other aspects of thedisclosure. Likewise, the term “aspects” does not require that allaspects of the disclosure include the discussed feature, advantage, ormode of operation. The term “coupled” is used herein to refer to thedirect or indirect coupling between two objects. For example, if objectA physically touches object B, and object B touches object C, thenobjects A and C may still be considered coupled to one another—even ifthey do not directly physically touch each other. For instance, a firstobject may be coupled to a second object even though the first object isnever directly physically in contact with the second object. The terms“circuit” and “circuitry” are used broadly, and intended to include bothhardware implementations of electrical devices and conductors that, whenconnected and configured, enable the performance of the functionsdescribed in the present disclosure, without limitation as to the typeof electronic circuits, as well as software implementations ofinformation and instructions that, when executed by a processor, enablethe performance of the functions described in the present disclosure.

It should be understood that the figures are for purposes ofillustration and that various elements are not to scale for ease ofunderstanding. Directional references such as “top,” “bottom,” “side,”“above,” “over,” “below” and similar references refer to the orientationof the figures unless explicitly stated and are not meant to require anyparticular orientation unless explicitly stated.

While conventional silicon and silicon-on-insulator devices areattractive for their low costs and ease of integration with otherubiquitous silicon-based semiconductor devices such as conventionalmicroprocessors, these devices have disadvantages. In particular,conventional silicon devices have limited maximum switching speeds andpoor power-handling. The operation frequency of a silicon MOSFET may betuned by varying structural details such as gate dielectric materialsand thicknesses. However, modifications that raise operating frequenciessuch as thinning the gate dielectric typically result in lower breakdownvoltages and other undesirable characteristics such as increasedgate-induced drain leakage (GIDL).

Compound semiconductor devices including III-V semiconductor devices(e.g., GaAs, GaN, et al.) can achieve higher operating frequencies andbetter power handling (i.e., greater current densities) in RF amplifierapplications. However, compound semiconductor manufacturing is not ascost effective as silicon-based semiconductor manufacturing. Andfurther, compound semiconductor devices are not readily integrated withpervasive silicon devices. Although silicon-on-insulator (SOI)technologies can improve performance through reduced substratecapacitance and other factors, SOI-based RF devices are still subject totrade-offs between operation frequency, breakdown voltages, and leakage.

Important performance characteristics of MOSFETs include maximumoperating frequency (as represented by a unity gain cut-off frequency),various breakdown voltages (e.g., gate-to-drain breakdown,gate-to-source breakdown, gate-to-well breakdown, drain-to-sourcebreakdown), on-state resistance, parasitic capacitance, and so on. ForMOSFETs used as amplifiers, derived performance characteristics such asthe product of the voltage gain and the operating bandwidth(“gain-bandwidth product”) and the product of the breakdown voltage andcut-off frequency (“BVCEO-Ft product”). Typically, the high gain comesat the expense of operating bandwidth and vice versa. Similarly, highbreakdown voltage typically comes at the expense of unity gain cut-offfrequency and vice versa. Doping choices and device geometry influencebreakdown voltages in addition to other parameters. Another component ofdevice breakdown is the gate dielectric. Thin gate dielectrics mayimprove on-state resistance, but they are more susceptible to breakdownat high voltage. Thus, in conventional MOSFETs, it is impossible toimprove device performance by thinning the gate dielectric withoutcompromising resistance to breakdown at high operating voltages. Otherperformance characteristics such as gate-induced drain leakage (GIDL)are also subject to trade-offs.

In conventional MOSFET structures, increased drain voltages may causethe depletion region formed at junction between the drain and channelregions to extend beneath the gate electrode, resulting in a shortenedeffective channel length, lower output resistance, and degraded gainwhen operated as an amplifier. It is therefore preferable to extend thedepletion region into the drain side drift region instead, therebyincreasing the breakdown voltage and increasing the output resistance,resulting in higher gain when the MOSFET is operated as an amplifier.

In conventional silicon MOSFETs, switching speeds may be increased byreducing the channel length. In order to avoid drain-induced barrierlowering (DIBL), the channel doping must be increased, which will tendto raise the threshold voltage (Vt) of the device. When the channeldoping is increased, the gate dielectric must be made thinner to avoidraising Vt. However, thinning the gate dielectric will typically resultin undesired effects such as lowered breakdown voltages and increasedGIDL.

Devices and methods disclosed herein allow for increased effectivechannel doping without the usual unwanted side effects of reducedbreakdown voltages, enabling silicon-based RF amplifiers, switches, andother devices that can realize higher operating frequencies thanconventional silicon devices (including SOI devices) without sufferingfrom the same compromises in terms of breakdown-free operation at highvoltages and other important characteristics. For example, devices andmethods disclosed herein can enable RF switches with reduced spuriousharmonic generation. Harmonics in MOSFET-based RF switches areassociated with gate-induced drain leakage (GIDL). Because GIDL isrelated to tunneling between energy bands, it increases exponentiallywith increased voltage. Silicon-based devices with a low GIDL andmethods for fabricating such devices according to embodiments disclosedherein are described further below. Such devices may be desirable fortheir low costs, reduced device sizes, and ease of integration withubiquitous wireless devices utilized in silicon semiconductortechnologies.

Some aspects of the present disclosure are discussed below withreference to example devices and methods for manufacturing such devices.Although certain example devices and methods disclosed herein arediscussed in the context of silicon-on-insulator technology, it shouldbe understood that the disclosed improvements may be applied tobulk-silicon-based devices and other device semiconductor platforms.

FIG. 1 is a cross-sectional schematic view of an examplesilicon-on-insulator device 100 suitable for use as a radio-frequency(RF) amplifier. The device 100 is a MOSFET fabricated on a SOI substrate110 having a buried oxide 112 with an n-type silicon body 120 above it.For purposes of illustration, the example device 100 and other devicesare shown as n-channel transistors having n-type source and drain (e.g.the source 130 and drain 135) and a p-type channel (e.g., the p-typewell 131 forming a channel). However, it will be appreciated thatmethods disclosed herein are equally applicable to fabrication ofp-channel transistors by substituting n-type doping for p-type dopingand vice versa as appropriate.

A source 130 may be formed by a highly-doped n-type (n+) well 132 formedwithin a p-type-doped p-type well 131, which forms a channel togetherwith the n-type body 120. A drain 135 is formed by a highly-doped n-type(n+) well. The device 100 may be gated by a gate 150 that includes gateelectrodes 152, 154, and 156, each separated from electrical contactwith the active regions of the device by one or more dielectrics. Thegate electrodes 152, 154, 156 may be formed from any combination ofmaterials with suitable electrical conductivity and other properties. Insome embodiments, the gate electrodes 152, 154, 156 may be formed fromhighly-doped (including degenerately-doped) polysilicon. Notably, thedielectric thickness between the active region(s) and gate 150 isdifferent beneath each of the gate electrodes 152, 154, 156. The firstdielectric material 140 and the second dielectric material 144 may beany suitable material(s) including, as non-limiting examples, silicondioxide, silicon nitride, cerium oxide, hafnium oxide, et al.). Thefirst dielectric material 140 may include one or more portions formed atdifferent times (e.g., as indicated by the dashed line in FIG. 3A and bythe spacers 343 of FIG. 3F).

As shown (and described further in connection with FIGS. 3A-E below),the gate electrodes 152, 156 are separated from the active regions bythe first dielectric material 140, while the gate electrode 154 nearestthe source 130 is separated from the p-type well 131 by a thinner,second dielectric material 144. A suitable electrically-conductivematerial 155 (e.g., any suitable metal or metal silicide) acts as a topcontact for gate 150 as well as for source 130 and drain 135. Example ofthe electrically-conductive material 155 is such as but not limited tosilicide formed with Titanium or Cobalt silicon diffusion salicidationprocess. Silicide with metal diffusion material such as Nickel is alsoviable with epitaxial growth of silicon that closes gaps between gatesprior to the silicidation process.

In some embodiments, the gate electrodes 152, 154, 156 may havesubstantially vertical sidewalls, forming an angle of approximately 90°with a surface of the active regions, allowing the gate electrodes 152,154, 156 to be used as masks for dopant diffusion during manufacturingof the device 100 (e.g., as described further in connection to FIGS.3A-E below).

Compared to a conventional laterally-diffused metal-oxide semiconductor(LDMOS) device, the greater thickness of the first dielectric material140 in the region 146 under the gate electrode 156 compared to thesecond dielectric material 144 can enable higher breakdown voltage byreducing the effective electric fields from the drain 135. Meanwhile,the relatively low thickness of the second dielectric material 144 canenable high gain without substantially compromising the ability of thedevice 100 to withstand high applied voltages. The dimensions of theexample device 100 may be chosen to obtain desired performancecharacteristics. Notably, using methods disclosed herein it is possibleto achieve a breakdown-voltage-cutoff-frequency product of at least 150GHz-Volts in a device similar to the example device 100. It will beunderstood that the thickness of the dielectric material 140 in theregion 146 under the gate electrode 156, the thickness of the dielectricmaterial 144, and the thickness of the dielectric material 140 in theregion 142 under the gate electrode 152 may be independently tuned tosuit various applications.

For instance, the use of a relatively thin dielectric material 144enables high transconductance. Meanwhile, choosing a larger thicknessfor the dielectric material 140 in region 146 relative to the thicknessof the dielectric material 140 in region 142 (together with the lateraldimensions of the gate electrode 152) can be used to improve thehigh-voltage endurance of the device near the drain 135. Typically, athickness of 20 nm for the dielectric material 140 in the region 146with proper drain engineering is sufficient for the example device 100to withstand applied drain-to-gate voltages of ˜20V when the dielectricmaterial 140 is silicon dioxide. Breakdown-free operation at highervoltages is also possible by further increasing the thickness of thedielectric 140 in the region 146 underneath the gate electrode 156.

The example device 100 is a modified n-channel laterally-diffusedmetal-oxide-semiconductor (LDMOS) transistor. When device 100 is in theactive mode, current may travel from the source 130 to the drain 135under an applied drain-source bias voltage. Since the current is carriedby electrons, device characteristics may be understood by considering anelectron current flowing from the source 130 to the drain 135. The gateelectrode 154 nearest the source 130 is separated from the p-typechannel well 131 by a thin second dielectric material 144 that may bemade significantly thinner than the thickness of the first dielectricmaterial 140 under the other gate electrodes 152, 156 in the region 142and the region 146, respectively. Breakdown voltage may be furtherimproved by separating the channel well 131 from the drain 135 with alightly n-doped body 120, resulting in a wide depletion region throughwhich electrons may drift to the drain well 135 when the device isoperated.

In some embodiments, the thin, second dielectric material 144 has athickness of or less than 20 Å. The high effective capacitance betweenthe gate electrode 154 nearest the source 130, and the channel well 131,can result in a higher carrier concentration there, resulting in loweron-state resistance compared to a conventional LDMOS FET with a uniformgate dielectric. While a similarly reduced on-state resistance may beachieved by thinning the gate dielectric of a conventional device, doingso would result in undesirable increases in leakage currents (due toGIDL) and undesirable decreases in blocking voltage.

Features of the example device 100 may be further understood usingexample parameters. For example, the breakdown voltage of the exampledevice 100 will depend on the depletion region formed near the drainunderneath the region 146. If the dielectric material 146 is silicondioxide with a thickness of 200 Å, and no lightly doped drain (LDD)implant is used for the drain 135, the dielectric breakdown voltage maybe expected to be at least 20V, allowing safe operation at 15 Vpeak. Adrift region is formed under the gate electrode 152. During active modeoperation in which Vd>Vg the drift region is depleted of carriers nearthe surface and carriers flow at depth away from the surface. As aresult, the effective gate dielectric thickness in the drift regionunder the gate electrode 152 is equivalent to the thickness of thedielectric material 140 in that region plus the depth of the depletedsurface region (modified by the appropriate dielectric constants). As aresult, the thickness of the dielectric material 140 in the region 142under the gate electrode 152 may be made thinner to improve the linearmode on-state conductance of the channel. In addition, the pn junctionformed between the body 120 and the p-well 131 is now gated, therebyenhancing the breakdown voltage and overall BVCEO of the parasitic npnbipolar transistor formed by the n-type source 132, the p-type well 131and the n-type body 120 (together with the n-type well at the drain 135)which partly limits the high-voltage handling capacity of the exampledevice 100. The doping of the p-well 131 may be in the range of 1.0E18cm-3, which is considerably higher than typical high-voltage LDMOSdevices thanks to the use of the thin dielectric material 144. The highdoping of the p-well 131 reduces the injection ratio and current gain ofthe parasitic BJT and thus improves the breakdown voltage of the exampledevice 100.

The dimensions of the example device 100 may be chosen such that thetotal drift length from the edge of the drain 135 to the edge of thep-well 131 is greater than four times the thickness of the body 120 inorder to nearly eliminate drain induced barrier lowering (DIBL) effects.As a non-limiting example, a thickness of the body region 120 may bechosen to be 500 Å and a length defining a drift region of ˜200 nm. Thelateral distance between the edge of the gate electrode 152 nearest thedrain 135 and the drain 135 may be chosen as 100 nm or any othersuitable value.

By way of example, a device similar to the example device 100 may beconfigured for use as an RF amplifier with a breakdown voltage between5V and 20V. The thickness of the body 120 in this example is 50 nm, witha buried oxide of 400 nm beneath it. A length of the first gateelectrode 152 between ˜200 and 1000 nm is desirable for certainapplications and will determine the breakdown voltage of the device.

The thickness of the dielectric material 142 under the central gate 152may be chosen to optimize the drain current at the onset of pinch-off,which partially determines the power handling capability of the device100. The thickness of the thin dielectric material 144 under the secondgate electrode 154 is selected thin for best performance now that highvoltage bias is shielded. The thickness of dielectric material in region146 on the third gate electrode 156 is selected to withstand the drainvoltages applied during operation of the device.

For certain applications, it is desirable for the cumulative width ofthe gate electrode 152 and 156 to be ˜4× the thickness of the body 120to suppress DIBL effects. The width of the gate electrode 154 and thedoping underneath it may be adjusted to set threshold voltage and totune the on-state resistance. The high-voltage tolerance of the device100 may be further increased by using a larger width of the gateelectrode 156 and underneath gate dielectric thickness.

FIG. 2 is a cross-sectional schematic view of an examplesilicon-on-insulator device 200 (a modified n-channel MOSFET) suitablefor use as an RF switch. Although the device 200 is similar to thedevice 100 and may be fabricated using many of the same processingsteps, it is optimized for use as a switch rather than an amplifier. Forthis reason, an ultrathin gate dielectric (e.g., the second dielectricmaterial 144 of FIG. 1 ) is not necessary under gate 254. Thus, inexample device 200, the gate 250 is separated into the central gateelectrode 252 and two symmetrically-located and dimensioned extensiongate electrodes 254, 256. The gate electrodes 252, 254, 256 may beformed from any combination of materials with suitable electricalconductivity and other properties. In some embodiments, the gateelectrodes 252, 254, 256 may be formed from highly-doped (includingdegenerately-doped) polysilicon. As will be described below, thedielectric material 240 is patterned to create a smaller gap between thecentral gate electrode 252 and the channel 220 than between theadditional gate electrodes 254, 256 and the channel 220. The gateelectrodes 252, 254, 256 may be electrically coupled (e.g., shorted toeach other) by electrically-conductive material 255 (e.g., theelectrically-conductive material 155) which may be any suitable metal ormetal silicide, and which may also be patterned to form electricalcontacts to the source 230 and the drain 235.

By way of example, a device similar to the example device 200 may beconfigured for use as an RF switch or an RF antenna tuner with abreakdown voltage between 5V and 20V. The thickness of the body 220 inthis example is 50 nm, with a buried oxide of 400 nm beneath it. Alength of the first gate electrode 252 between ˜50 and 1000 nm isdesirable for certain applications. The thickness of dielectric materialin the regions 244, 246 under the second and third gate electrodes 254,256, respectively is selected to withstand the drain voltages appliedduring operation of the device and to reduce harmonics due to GIDL thusa thickness higher than 252. For these purposes the thickness of 20 nmfor the dielectric layer 240 in the regions 244, 246 may be suitable.For certain applications it is desirable for the cumulative width of thegate electrodes 250 to be ˜4× the thickness of the body 220 to reduceDIBL effects.

Suitable gate electrodes (e.g., the gate electrodes 252, 254, 256) maybe polysilicon with a thickness of ˜200 nm, with a spacing of 20 nmbetween each of the gate electrodes 254, 256 and the gate electrode 252.

In the device 200, the gap between the central gate 252 and theextension gate electrodes 254, 256, considered in isolation might beexpected to increase on-state resistance. However, when a conventionalFET is over-driven (i.e., Vg>>Vth), carrier mobility often suffers. Whenthe device 200 is over-driven, the dielectric thicknesses under thegates 254, 256 may be chosen such that the carrier mobility remainsnormal under the regions 244, 246, the low GIDL current resulting in lowharmonic distortion and higher breakdown voltage than in a conventionaldevice with a modest increase in on-state resistance (˜10-15%) whencompared to a conventional transistor without the extension gates 254,256 as described herein.

FIGS. 3A-3F are cross-sectional schematic views of an example device 300at selected points during an exemplary manufacturing process, providedto illustrate steps in an example process suitable for manufacturing anexample device 300 (e.g., the example device 100 of FIG. 1 ). WhileFIGS. 3A-3F illustrate fabrication of a device suitable for use as an RFamplifier, it should be understood that the methods disclosed herein areapplicable to other devices and other semiconductor technologies (e.g.,non-SOI silicon-based devices, compound semiconductor devices, etc.),with appropriate modifications, as discussed below.

As shown in FIG. 3A, a semiconductor substrate 310 is provided. Forpurposes of illustration, the substrate 310 is shown as an SOI waferwith a buried oxide layer 312 and a silicon body 320 over the buriedoxide 312. As shown, the semiconductor substrate 310 may be providedwith a gate electrode 352 formed above the region body 320 to form whatwill become part of a gate structure 350 (e.g., the gate 150 of FIG. 1). As shown the gate electrode 352 is surrounded by a dielectricmaterial 340. In some embodiments, the substrate 310 may be providedwith only a lower dielectric material 340A forming the lower portion ofthe first dielectric material 340 present (denoted by dashes), in whichcase an upper dielectric material 340B may be formed later. Thesubstrate 310 is shown as a partially-depleted SOI wafer. However, insome embodiments, a fully-depleted SOI wafer may be used.

In some embodiments, the substrate 310 may be provided without the firstdielectric material 340 and without the gate electrode 352. In suchembodiments, a method may include forming the lower portion 340A of thefirst dielectric material 340 and the gate electrode 352 using anysuitable method. The first dielectric material 340 may be any suitablematerial including, as non-limiting examples, various oxides andnitrides (e.g., silicon dioxide, hafnium oxide, cerium oxide, siliconnitride, boron nitride, et al.) and combinations thereof.

As shown in FIG. 3C, the first dielectric material 340 may be patternedvia photolithography and etching, or any other suitable combination ofprocesses to expose the region 325 on the top surface 322 of thesubstrate 310 next to the gate electrode 352. In some embodiments, thefirst dielectric material 340 is silicon dioxide and may be patterned byetching areas exposed after photolithographic resist development using asolution of hydrofluoric acid (HF). As shown, the gate electrode 352 maybe used as a hard mask for a dopant implantation 362. As shown, thedopant implantation 362 may create a p-type well 331 within the body320, which may be lightly n-doped. As will be discussed, part of thisvolume may form a source well 330 of a completed device 300 (asdescribed further below in connection with FIG. 3F). The dimensions ofthe p-type well 331, or at least a portion of its dimensions, may thusbe self-aligned to the nearest edge(s) of the gate electrode 352.

Due to carrier mobility with the silicon material, the compact devicesize is necessary to yield desirable high speed RF performance. Priorarts feature split gate structure form split gate oxides followed byaligning a gate structure over the split gate oxide through commonalignment target, which render device miniature difficult. The proposedmethod places the alignment over gate 152 as shown in FIG. 3C toeffectively eliminate the alignment tolerance burden on device sizereduction.

As shown in FIG. 3C, a thin dielectric material 344 may be formed overthe region 325. In some embodiments the thin dielectric material 344 maybe silicon dioxide formed using a thermal oxide process. The thicknessof the second dielectric material 344 may be chosen to achieve desiredperformance characteristics of the device 300. In particular, since thedielectric material 344 will form the gate dielectric between the gateelectrode 354 and the body 320 of the finished transistor in the region325 (as will be described further below), that thickness will at leastpartially determine the on-state resistance of the device 300 and itscut-off frequency. As shown, the dielectric material 340 may bepatterned such that the thickness of the dielectric material 340 isthicker in the region 346 than in the region 344.

In some embodiments, thicknesses as low as realistically achievablewithout causing shorting may be desirable. Device performance can betuned to optimize for different characteristics. For instance, as thedielectric material is made thinner, the drive current will increase.Similarly, as the thickness of the dielectric material 344 is increased,the drive current will be decreased. Any suitable thicknesses of thevarious dielectric materials may be chosen depending on characteristicsdesired for a particular application.

For most applications, an oxide layer having a thickness of 16-100 Å maybe used for the dielectric material 344. It will be appreciated thatother materials, including high-K dielectrics, and any other suitabledielectric may be used to further tune desired performancecharacteristics.

As shown in FIG. 3D, an electrically-conductive layer 353 may bedeposited over the structure of FIG. 3C and patterned to form thestructure in FIG. 3E. The conductive layer 353 may be any suitablematerial including metals and/or highly-doped polysilicon. Notably, thethickness of the-conductive layer 353 may be chosen such that theelectrically-conductive layer 353 can be patterned using anisotropicetching without the need for a separate lithographic step. By choosing asuitably anisotropic etching process (e.g., reactive ion etching at lowpressures as a non-limiting example) the portions of theelectrically-conductive layer 353 above the gate electrode 352 and abovethe region 325, may be removed while leaving portions of the layer 353along the sidewall 355 of the gate electrode 352 and the sidewall 357 ofthe patterned dielectric material 340 intact, thereby forming theextension gate electrodes 354, 356 (e.g., the gate electrodes 154, 156of FIG. 1 ). Such non-lithographic processes may be used to produce finenanometer-scale structures without requiring the expense of additionalmasks and equipment capable of nanometer (or sub-nanometer) maskalignment. An over etch required to clear layer 353 residue may causeextension gate electrodes 354, 356 height lower than central gateelectrode 352.

In some embodiments, to aid in subsequent patterning of the structure ofFIG. 3D to form the structure of FIGS. 3E and 3F, a thin oxide layer(e.g., the oxide layer 359) or other layer may be deposited over theelectrically-conductive layer 353 as shown in FIG. 3D. When subject to asuitably anisotropic selective etching process (e.g., low-pressureetching in HBr:Cl plasma) that preferentially removes material in adirection perpendicular to the surface of the substrate, this oxidecoating protects the sidewalls of the electrically-conductive layer 353from etching, resulting in the substantially vertical sidewalls 355A,Band 357A,B of the respective extension gate electrodes 354,356 as shownin FIGS. 3E-3F. The protective oxide coating ensures vertical gateelectrodes 354, 356 has a consistent dimension, are not shorted to thesource or drain doped well in the volume production.

It will be appreciated that the descriptions herein are for the purposesof illustrating key features of embodiments disclosed herein and mayomit various well-known processing steps. For instance, in someembodiments where polysilicon is used as a gate electrode material, aliner oxide may be deposited or thermally grown as part of a polysiliconannealing process to drive diffusion of heavy n-type dopants into thepolysilicon. Such liner oxide layers may be ˜50-200 Å thick and may beremoved by RIE before etching the polysilicon to pattern the gateelectrodes. This allows for control of critical dimensions (“CDcontrol”). Subsequent formation of the dielectric spacers 343 formationprevent shorting of the source and drain to the respective gateelectrodes 354, 356 during silicide formation.

As shown in FIG. 3F, dielectric spacers 343 may be patterned adjacent tothe extended gate electrodes 354, 356. The dielectric spacers 343 may beused as a hard mask for a dopant implantation process 364 that may beused to form source and drain wells. As shown the source and drain wellsare doped n+. The extents of the source well 332 and drain well 335, aswell as the interface between the source well 332 and the channel well331, and between the drain well 335 and the body 320, may be furtherdefined by one or more thermal annealing steps. In some embodiments inwhich the gate electrodes 354, 356 are polysilicon, one or moreannealing steps may also be used to diffuse dopants within the gateelectrodes 354, 356 to ensure that they have sufficient electricalconductivity. In some embodiments, a thermal annealing step may be usedto repair etching damage introduced by reactive ion etching of the gateelectrodes 354, 356 and additional annealing to further diffuse dopantsintroduced by the doping process 364 and/or the doping process 362. Insome embodiments, the doping profiles of the source, drain, and channelare configured to transition gradually at the interface between thechannel well 331 and the body 320, thereby reducing high field gradientsassociated with abrupt junctions.

Additional conventional steps not shown explicitly may include lateraldopant diffusion to form the arrangement of the doped source and drainregions shown in FIG. 3F, and deposition and patterning of the spacers343, as well as deposition of source and drain contacts which may beformed from a silicide, metal, or any other suitable material. It willbe understood that geometries of the doped regions pictured will bealtered if fully-depleted SOI is used; for example, the p-type well 331and the n-type well forming the drain 335 may extend to the bottom ofthe body 320 at the interface with the buried oxide 312.

FIG. 4 shows a flow diagram illustrating an example process 400 forfabricating a transistor (e.g., the example device 100). As describedbelow, some or all illustrated features may be omitted in a particularimplementation within the scope of the present disclosure, and someillustrated features may not be required for the implementation of allembodiments. The example process 400 has steps 402, 404, 406, 408, 410,412, 414, 416. These steps may be performed by a human operatingsemiconductor fabrication equipment, automated control systems operatingsuch equipment, or by any combination thereof, described for purposes ofillustration as a single “operator.” In some examples, process 400 maybe carried out by any suitable apparatus or means for carrying out thefunctions or algorithm described below. In some examples, the sequenceof steps 406 and 408 may be reversed to allow implantation through theoxide.

At step 402, an operator provides a substrate having a semiconductiveregion (e.g., body 320 of FIG. 3 ) extending below a top surface of thesubstrate. The substrate has first and second ends (e.g., the source 330and drain 335 of FIGS. 3A-3F) opposing one another along a directionparallel to the top surface of the substrate; a first dielectricmaterial disposed over the semiconductive region (e.g., the lowerdielectric material 340 Å of FIG. 3 ); and a first gate electrode (e.g.,the gate electrode 352 of FIG. 3 ) disposed over the semiconductiveregion between the first and second ends.

At step 404, an operator forms a second dielectric material (e.g., theupper dielectric material 340B of FIG. 3 ) over the first gate electrodeand the substrate. The second dielectric may be silicon dioxide, siliconnitride, or any other suitable dielectric or combinations thereof andmay be deposited by sputtering, physical vapor deposition, chemicalvapor deposition, or any other suitable method.

At step 406, the operator patterns the first and second dielectricmaterials to expose a first region of the top surface of the substrate(e.g., the region electrode 325 as shown in FIG. 3C) adjacent to atleast a first sidewall of the first gate electrode and near the firstend. In some embodiments, patterning the second dielectric materialincludes first patterning photoresist to create an etch mask. Theexposed areas of the second dielectric material as well as the firstdielectric material within the first region may be removed using anysuitable wet or dry etching process. As one non-limiting example,silicon dioxide may be removed by using a solution of hydrofluoric acid.

At step 408, the operator applies a first dopant (e.g., the dopant 362shown in FIG. 3C) to the substrate to form a first doped volume within afirst volume of the substrate corresponding to the first region. Becausethe substrate is exposed within the first region, the first doped volumehas a width determined at least in part by a width of the first region.The first dopant may be applied using any suitable process(es) includingdiffusion of dopant from a coated layer or ion implantation. If thetransistor is an n-channel transistor, the first dopant is chosen tocreate a p-doped region. If the transistor is a p-channel transistor,the first dopant is chosen to create an n-doped region.

At step 410 the operator forms a third dielectric material (e.g., thethin dielectric material 344 shown in FIG. 3C) on the first region witha thickness less than the thickness of the first dielectric materialbetween the first gate electrode and the top surface of the substrate.The third dielectric may be silicon dioxide, silicon nitride, or anyother suitable dielectric or combinations thereof and may be depositedby sputtering, physical vapor deposition, chemical vapor deposition, orany other suitable method. In some embodiments, the third dielectricmaterial may be formed on the first region by using thermal oxidation ofthe top surface of a substrate in the first region.

At step 412, the operator forms a second gate electrode by patterning anelectrically-conductive layer (e.g., conductive layer 353 shown in FIG.3D, patterned as shown in FIG. 3E to form the two extension gateelectrodes 354 and 356). The electrically conductive layer may bedeposited by any suitable processes (e.g., chemical vapor deposition,sputtering, etc.) The second gate electrode is near the first end of thesemiconductive region and disposed adjacent to at least the firstsidewall of the first gate electrode and disposed over at least aportion of the second dielectric material and the first doped volume. Insome embodiments, the second gate electrode may be patterned without theneed to perform a lithographic step. For instance, as a non-limitingexample, the electrically-conductive layer may be etched via highlyanisotropic etching process such as reactive ion etching (RIE) thateffectively removes the bulk of the electrically-conductive layer whileleaving one or more portions of the electrically-conductive layer intactalong sidewalls of the first gate electrode. The width of the secondgate electrode may be controlled by choosing an initial thickness of theelectrically-conductive layer.

The second gate electrode may be made as narrow as desired down to atleast the scale of tens of nanometers without the need to performhigh-resolution lithography and high-precision mask alignment. This stepmay optionally include forming a third gate electrode opposite thesecond gate electrode along an opposite sidewall of the first gateelectrode (and near the second end of the semiconductive region). Thesecond dielectric material need not be removed in the area correspondingto the third gate electrode. As a result, the third gate electrode maybe separated from the semiconductive region by a greater thickness ofdielectric material than the first gate electrode and/or the second gateelectrode (see FIG. 1 for example). A significant benefit of the thickdielectric under the third gate electrode is reduced capacitance betweenthe overall gate structure and the drain (“Cgd”). Reduced Cgd enableshigher power gain and higher maximum operation frequencies (as measuredby the highest frequency at which the gain is greater or equal to unitypower gain). The extended gate structure enables high speed performancewithout power gain degrade due to high gate resistance Rg seen withconventional narrow gate structure.

At step 414, the operator forms a first dielectric spacer adjacent tothe second gate electrode (e.g., one of the dielectric spacers 343adjacent to the second gate electrode 354 as shown in FIG. 3F) bypatterning a fourth dielectric material. The fourth dielectric materialmay be any suitable dielectric as previously described and may bedeposited and patterned using any suitable process, includingreactive-ion etching such as described above in connection to step 412.The first spacer may be used as a hard mask for application of a dopantwhich may be used to form a source well within the first doped volume.Accordingly, at step 416, the operator applies a second dopant to thesubstrate to form a second doped volume (e.g., the n+ well 332 shown inFIG. 3F) within the first doped volume of the substrate. The seconddoped volume has a width determined by at least a width of the firstregion, a position of the second gate electrode, and a width of thefirst spacer (see, for example, FIG. 3F and descriptions thereof). Step414 may also include forming a second spacer adjacent to the third gateelectrode in embodiments that include forming a third gate electrode asdescribed above. In such embodiments, the second spacer may be used asmask through which the second dopant forms a third doped region usableas a drain of the transistor (e.g., see the third gate electrode 356,the spacers 343, and the drain 335 of FIG. 3F). The second dopant ischosen to introduce an opposite majority carrier type as the firstdopant such that the source has opposite doping to the channel formedunderneath the second gate electrode.

In a further aspect of this disclosure, methods disclosed herein may bebut not limited to non-SOI based devices, e.g., as illustrated by FIGS.5A-5C. FIG. 5B shows an example device 500 suitable for use as a lowloss power management device having similar features to the exampledevice 100 of FIG. 1 , fabricated using a conventional bulksemiconductor substrate 510. A SOI version is also desirable for compactdevice isolation and high speed switching to reduce external passivecomponents size. The source well is formed by an n+ doped well 533formed within a p-doped well 532. A p+ doped well 534 forms an Ohmiccontact to p-doped well 532 and connect the n-doped source well 533. Theextended gate electrode 554 width may be greater than 0.1 um.

A parasitic npn bipolar junction transistor is formed by the n-type well533, the p-type well 532, and the n-type substrate 510. Leakage currentunder high voltage operation across the pn junction formed between thep-type body 531 and n-type body 510 flow toward the p-well contact 534acts as a base current in the parasitic BJT. In a conventional devicewith a single thick gate dielectric, doping of the p-well 532 is usuallyapproximately 5.0E16 cm-3 to achieve an acceptable threshold voltage.However, the high p-well resistance may forward bias the pn junctionformed between the p-type well 532 and the n-type source well 533,causing failure. Using an extended gate electrode 554 enabled using athin dielectric material 544, allows much higher dopant concentrationsin p-type well 532 (e.g., ˜1.0E17-1.0E18 cm-3) while retaining low onstate resistance (Ron). This reduces the current gain of the parasiticBJT, thereby increasing BVCEO and overall high voltage handlingcapacity.

Further, the deep lightly-doped p-type body well 531 can isolate thesource well 530 from the substrate 510, which may be doped lightlyn-type. The lightly-doped p-type well 531 increases device breakdownvoltage with a suitably large depletion region at the junction with thebackground doping of the substrate. Meanwhile, the thin dielectricmaterial 544 enabled by methods disclosed herein allows the p-type well532 to be heavily-doped, allowing a low-resistance path for reverse-biascurrent.

As shown, the n+ doped drain well 535 is coupled to the source well 530through the lightly-doped substrate 510 and the p-type channel well 532.In this arrangement, the portion of the substrate 510 through whichcarries flow when the device is operated functions as a drift region,which acts as a depletion mode transistor channel when positively biasedvia the gate 550 which comprises a first gate electrode 552 and a secondgate electrode 554 (similar to the gate electrodes 152, 154 of thedevice 100 of FIG. 1 ). The isolation trench 557 may be used to protectgate from high drain voltage where conventional thermal local oxidation(LOCOS) is also preferred for low cost. A large depletion region isformed between the deep p-body 531 and the n-substrate 510. Theisolation trench reduces field concentration at the drain. Current isconcentrated toward the channel which, together with a thin gatedielectric over the channel reduces the on-state loss while maintainingthe high-voltage endurance of device.

Similar to device 100, device 500 has a thin dielectric material 544between the gate electrode 554 and the effective channel, and adielectric material 540 in the region 542 with a greater thickness thanthat of the dielectric material 544 between the gate electrode 552 andthe drift region below it. The gate electrodes 552, 554, and the sourcewell 530 and drain well 535 may be provided with electrical contacts bypatterning a suitably electrically conductive layer 555 (e.g., asuitable metal or metal silicide). For purposes of illustration, theexample device 500 is shown as an n-p-n transistor having n-type sourceand drain wells, and a p-type channel well. However, it will beappreciated that methods disclosed herein are equally applicable tofabrication of p-n-p transistors with a p-type channel by substitutingn-type doping for p-type doping and vice versa as appropriate.

FIG. 5A depicts an example doping procedure useful in the fabrication ofthe structures shown forming the source well 530 in FIG. 5B. As shown, alayer of photoresist 559 is patterned over the structure including thedrain well 535 (not shown), exposing the source well 530. An angleddopant implantation 560 may be used to form the deep p-doped body well531. A second dopant implantation 565 may be used to form the shallower,more highly doped p-well 532. Additional deep implant may be beneficialin further reduce parasitic path resistance. After the resist 559 isremoved, similar processes to those described above in connection toFIGS. 3A-3F may be used to complete the fabrication of the device 500.The extension gate 556 and thick dielectric 546 may replace trench formore compact device shown in FIG. 5C. Additional mask may be patternedto block p-well 532 implant in drain area 535.

One or more of the components, steps, features, and/or functionsillustrated in FIGS. 1-5 may be rearranged and/or combined into a singlecomponent, step, feature, or function or embodied in several components,steps, or functions. Additional elements, components, steps, and/orfunctions may also be added without departing from novel featuresdisclosed herein. The apparatus, devices, and/or components illustratedin FIGS. 1-5 may be configured to perform one or more of the methods,features, or steps described herein.

It is to be understood that the specific order or hierarchy of steps inthe methods disclosed is an illustration of exemplary processes. Basedupon design preferences, it is understood that the specific order orhierarchy of steps in the methods may be rearranged. The accompanyingmethod claims present elements of the various steps in a sample order,and are not meant to be limited to the specific order or hierarchypresented unless specifically recited therein.

The previous description is provided to enable any person skilled in theart to practice the various aspects described herein. Variousmodifications to these aspects will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother aspects. Thus, the claims are not intended to be limited to theaspects shown herein, but are to be accorded the full scope consistentwith the language of the claims, wherein reference to an element in thesingular is not intended to mean “one and only one” unless specificallyso stated, but rather “one or more.” Unless specifically statedotherwise, the term “some” refers to one or more. A phrase referring to“at least one of” a list of items refers to any combination of thoseitems, including single members. As an example, “at least one of: a, b,or c” is intended to cover: a; b; c; a and b; a and c; b and c; and a, band c. All structural and functional equivalents to the elements of thevarious aspects described throughout this disclosure that are known orlater come to be known to those of ordinary skill in the art areexpressly incorporated herein by reference and are intended to beencompassed by the claims. Moreover, nothing disclosed herein isintended to be dedicated to the public regardless of whether suchdisclosure is explicitly recited in the claims. No claim element is tobe construed under the provisions of 35 U.S.C. § 112(f) unless theelement is expressly recited using the phrase “means for” or, in thecase of a method claim, the element is recited using the phrase “stepfor.”

Although embodiments have been described herein with respect toparticular configurations and sequences of operations, it should beunderstood that alternative embodiments may add, omit, or changeelements, operations and the like. Accordingly, the embodimentsdisclosed herein are meant to be examples and not limitations.

What is claimed is:
 1. A method for fabricating a low loss power devicecomprising the steps of: providing a substrate having a semiconductiveregion extending below a top surface of the substrate, thesemiconductive region having first and second ends opposing one anotheralong a direction parallel to the top surface of the substrate; forminga first dielectric layer above the semiconductive region; disposing afirst gate electrode over the semiconductive region between the firstand second ends; forming a third dielectric layer over the first gateelectrode and the first dielectric layer; patterning the first and thirddielectric layers to expose a first region of the top surface of thesubstrate; forming a second dielectric layer having a second thicknesson the first region of the top surface of the substrate adjacent to thefirst dielectric layer below the first gate electrode and near the firstend; and disposing a second gate electrode over the second dielectriclayer.
 2. The method according to claim 1, further comprising a step of:applying a first dopant to form a first doped volume within a firstvolume of the substrate corresponding to the first region, the firstdoped volume having a width determined at least in part by a width ofthe first region.
 3. The method according to claim 2, further comprisingthe steps of: forming a first spacer adjacent to the second gateelectrode; and applying a second dopant to form a second doped volumewithin the first doped volume of the substrate, the second doped volumehaving a width determined by at least a width of the first region, aposition of the second gate electrode, and a width of the first spacer;wherein the second doped volume has a majority carrier type opposite toa majority carrier type of the first doped volume.
 4. The methodaccording to claim 1, wherein the second gate electrode is formed by thesteps of: forming an electrically-conductive layer above the first gateelectrode and the first region; and patterning theelectrically-conductive layer by an anisotropic reactive ion etching(RIE) process that leaves behind a portion of theelectrically-conductive layer on the second dielectric layer to form thesecond gate electrode, the second gate electrode having verticalsidewalls that are substantially perpendicular to the top surface of thesubstrate.
 5. The method according to claim 4, wherein theelectrically-conductive layer is patterned after forming above the firstgate electrode and above the first region without forming any layer thatacts as a mask defining dimensions of the second gate electrode; and awidth of the second gate electrode is defined by an as-formed thicknessof the electrically-conductive layer.
 6. The method according to claim4, wherein patterning the electrically conductive layer comprises thesteps of: forming a protective dielectric material layer above theelectrically-conductive layer with a protective layer thickness; andetching the protective dielectric material layer using the anisotropicRIE process; wherein the anisotropic RIE process preferentially etchesthe protective dielectric layer and the electrically-conductive layeralong a direction perpendicular to the top surface of the substrate;wherein the anisotropic RIE process removes the protective dielectriclayer with a greater etching rate than an etching rate for theelectrically-conductive layer; wherein the protective layer thicknessand the anisotropic RIE process are jointly configured such thatresidual protective dielectric material adheres to a vertical sidewallof the second gate electrode farthest from the first gate electrode thatfaces the first gate electrode; and wherein the protective layerthickness and the anisotropic RIE process are jointly configured suchthat residual protective dielectric material adheres to a verticalsidewall of the third gate electrode farthest from the first gateelectrode that faces the first gate electrode.